Object oriented applications usually have inheritance as an important part of their design, including in their domain objects. However, the corresponding data model has no built-in mechanism for specifying inheritance. Therefore, you must map your domain objects to your relational database intelligently.
Domain objects in an application represent the core data that is used by the application and therefore these objects are usually persisted in some data source. If the data source is a relational DBMS, then domain objects have to be mapped to a relational model which looks different from an object model. Therefore, there are many things that you must keep in mind when mapping these domain objects to a relational database. This article focuses on mapping inheritance in your domain objects to your relational databases.
Read the complete article at http://www.alachisoft.com/rp.php?dest=/articles/inheritance_mapping.html